1. Field of the Invention
The present invention relates generally to a storage control apparatus, method and program for a disk array, etc., securing data redundancy through RAID configuration and, more particularly, to a storage control apparatus and control method and program assuring data recovery based on the redundancy configuration upon failure disconnection of a storage device.
2. Description of the Related Art
In a conventional disk array system, data redundancy has been enhanced by configuring RAID (Redundant Array of Independent Disks) composed of a plurality of disk devices arranged in a disk array, to respond to I/O requests from a host. Though various types of RAIDs exist, RAID5 has ordinarily been used that is suited for I/O requests from the host attendant on the transaction processing. The RAID5 writes user data into (N-1) disk devices having the same logical block address making up a stripe, of N disk devices configuring a disk array, and writes a parity into the remaining one, the parity being generated by EXORing the user data, the disk device with the parity written thereinto being different from stripe to stripe so that the parities can be distributed. In the disk array system having such a RAID5 redundant configuration, when a disk device fails and degenerates, the user data of the failed disk device can be recovered by implementing the EXOR operation of the user data and the parity read from the other disk devices forming a RAID group together with the failed disk device.
Such a conventional RAID5 disk array system, however, may face the worst situation possible that the data cannot be recovered and goes lost due to being incapable of the EXOR operation in case, when a disk device fails and degenerates, data cannot be read from two or more disk devices including the failed one. The following may be considered as causes of rendering the data recovery infeasible upon the occurrence of a failure in the disk device. First, is a case where the parity consistency has broken down as a result of the parity going abnormal due to some reasons such as design errors of firmware of the disk device. There is another case where an abnormality has occurred in a medium of the other disk device than the failed disk device. Furthermore, a portion unsubjected to read and write for a long while may appear even at the same logical block address on the same stripe, and such a portion may not probably undergo data recovery due to the medium abnormality occurring thereat.